Cisplatin (DDP) is one of the most widely used antineoplastic agents for the treatment of human ovarian cancer (Ozois and Young, Semin. Oncol. 11:251-263 (1984); Hakes, et al., Proc. Am. Sci. Clin. Oncol. 8:152 (1989)). In spite of its potency, the frequent development of DDP resistance is a major obstacle to curative therapy (Chu, et al., Proc. Am. Assoc. Cancer Res. 30:594 (1989); Meitner, et al., Proc. Am. Assoc., Cancer Res. 30:508 (1989)). Although the mechanism of DDP resistance in vivo is not characterized, a good deal of information is available about mechanisms in cell lines. Impairment of DDP uptake is one of the important mechanisms contributing to DDP resistance (Andrews, et al., Cancer Res. 48:68-73 (1988); Waud, Cancer Res. 47:6549-655 (1987); Richon, et al., Cancer Res. 47:2056-2061 (1987); Teicher, et al., Cancer Res. 47:388-393 (1987)). Increased levels of metallothioneins have also been reported in some (Kelley, et al., Science 241:1813-1815 (1988)), but not all (Andrews, et al., Cancer Chemother. Pharmacol. 19:149-154 (1987)), DDP-resistant cells. Kelley et al. reported that, in a murine leukemia cell line, the degree of resistance was proportional to metallothionein content, and that loss of resistance to DDP in a revertant cell line was associated with concomitant lowering of metallothionein content. Modulation of glutathione (GSH) concentrations in mammalian cells has also been reported to influence the cytotoxicity in DDP (Lee, et al., Cancer Res. 48:3661-3665 (1988)). Some DDP-resistant cell lines contain increased amounts of glutathione (Rice, et al., Cancer Res. 46:6105-6110 (1986)), and DDP sensitivity can be enhanced under some conditions by extensive depletion of glutathione by buthionine sulfoximine (Andrews, et al., Mot. Pharmacol. 30:643-650 (1986)). DDP reacts with DNA to produce an intra-strand N7d(GpG) diamine platinum adduct which comprises 40-60% of the platinum bound to DNA (Poivier, et al., Proc. Natl. Acad. Sci. U.S.A. 79:6443-6447 (1982)). DNA repair defective cells are hypersensitive to DDP (Fox and Roberts, Cancer Metastasis Rev. 6:261-281 (1987)), and enhanced DNA repair has been implicated in the DDP-resistant phenotype (Masuda, et al., Cancer Res. 48:5713-5716 (1988)).
Besides cisplatin, other chemotherapeutic agents exist, including such compounds as carboplatin, tetraplatin, taxol, melphalan, and 5-fluorouracil, all of which exhibit severe toxic side effects that limit somewhat their clinical usefulness. These additional agents exhibit various modes of action and it would be of significant utility to increase their cytotoxicity toward cancer cells while not increasing overall systemic toxicity.
The tumor-promoting phorbol diester TPA (12-O-tetradecanoyl phorbol-3-acetate) has profound effects on a variety of cellular functions depending upon the type of cell treated, including either stimulation (Diamond, et al., Int. J. Cancer 13:721-730 (1974); Dridger and Blumberg Cancer Res. 37:3257-3265 (1977)), inhibition (Diamond, et al., Nature 269:247-249 (1977)); Cohen, et al., Nature 266:538-540 (1977)) of proliferation, or induction of differentiation (Dridger and Blumberg Cancer Res. 37:3257-3265 (1977); Diamond, et al., Nature 269:247-249 (1977)). The effects of TPA appear to be largely mediated through stimulation of protein kinase C and the subsequent phosphorylation of a variety of protein substrates (Blacksher, et al., FASEB 2: 2957-2969 (1988); Nishizuka, Nature 308:693-698 (1988)).
TPA has been reported to alter cellular sensitivity to several kinds of antineoplastic agents. In the human KB carcinoma cells, TPA treatment decreased sensitivity to etoposide and vincristine by 50%, but this effect could not be mimicked by treatment of cells with 1-oleoyl-2-acetylglycerol (OAG), calling into question a role for protein kinase C (Ferguson and Cheng, Cancer Res. 47:433-441 (1987)). Posada (Posada, et al., J. Biol. Chem., in press (1990)) demonstrated that TPA enhanced the cytotoxic activity of doxorubicin in sarcoma 180 cells. Conversely, down regulation of protein kinase C produced by long term exposure to TPA resulted in a decreased cytotoxic effect of doxorubicin (Posada et al., J. Biol. Chem., in press (1990)). Protein kinase C can phosphorylate the MDR 1 gene product, which functions as efflux pump for etoposide, vincristine, and doxorubicin. Hofmann (Hofmann, et al., Int. J. Cancer 42:382-388 (1988)) reported that either inhibition of protein kinase C activity with a series of compoundation such as quercetin (3,3', 4'5,7 -pentahydroxyflavone), tamoxifen, staurosporine, either lipid analog (BM4144026) (Hofmann, et al., Lipids 24:312-317 (1989)), or down regulation with long term exposure to TPA enhanced the sensitivity of cells to DDP.
Platinum complexes, particularly cisplatin (DDP), are drugs of major importance in cancer therapy. However, both intrinsic and acquired resistance to DDP occurs frequently. Cells selected for resistance to many antimetabolites (Kaufman, et al., Proc. Natl. Acad. Sci. U.S.A. 76:5669 (1979)), and to drugs participating in the multiple drug resistance phenotype (Gottesman and Pastan, J. Biol. Chem. 263:12163 (1988)), often exhibit very high levels of resistance. In contrast, both in vitro and in vivo selection with DDP at clinically relevant intensities usually results in cells only 2 to 4-fold resistant to this drug (Wilson, et al., Br. J. Cancer 56: 763 (1987)). Attempts to enhance the sensitivity to DDP have focused on strategies such as reduction of glutathione (Andrews, et al., Chem. Biol. Interactions 65:51 (1988)) and inhibition of DNA repair (Hamilton, et al., Proc. A. Assoc. Cancer Res. 28:291 (1987)).